Scoring of glass is conventionally accomplished using mechanical tools. However, an alternative exists that uses laser radiation, e.g., CO2 laser radiation at a wavelength of 10.6 μm, to heat the glass and create tensile stress via a temperature gradient. The use of a laser for glass scoring is discussed in commonly-assigned U.S. Pat. No. 5,776,220 entitled “Method and apparatus for breaking brittle materials” and U.S. Pat. No. 6,327,875 entitled “Control of median crack depth in laser scoring,” the contents of both of which are incorporated herein by reference in their entireties.
As shown in FIG. 1, during laser scoring, a median crack (also known as a partial vent or, simply, a vent) is created in a major surface 114 of a glass sheet or ribbon 112 along a score line 115. In order to create the vent, a small initiation flaw 111 is formed on the glass surface near one of its edges, which is then transformed into the vent by propagating a laser light beam 121 having a footprint 113 across the surface of the glass followed by a cooling area produced by a cooling nozzle 119. Heating of the glass with a laser light beam and quenching it immediately thereafter with a coolant creates a thermal gradient and a corresponding stress field, which is responsible for the propagation of the initiation flaw to form the vent.
Commonly-assigned U.S. Patent Publication No. 2008/0264994 (the '994 publication), the contents of which are incorporated herein in their entirety, describes a system for laser scoring of a moving glass ribbon in which a traveling carriage moves along a linear track which is inclined at an angle α with respect to a line transverse to the direction of motion of the ribbon.
FIGS. 2 and 3 of the present application schematically illustrate the system of the '994 publication. In this figure, the glass ribbon is identified by the reference number 13, the traveling carriage by the number 14, the linear track by the number 15, the support structure (support frame) for the track by the number 11, and the equipment which produces the ribbon, e.g., a fusion draw machine, by the number 9. As discussed in the '994 application, as seen from a fixed reference frame (e.g., the xyz reference frame in FIG. 2), the glass ribbon moves in the direction of vector 16 at a speed Sglass and the carriage moves in the direction of vector 17 at a speed Scarriage, where Sglass, Scarriage, and the angle α satisfy the relationship:Scarriage=Sglass/sin α.
In this way, the carriage keeps pace with the ribbon, or, more precisely, the magnitude of the component of the carriage's velocity that is parallel to the direction of motion of the ribbon equals Sglass. Consequently, as seen from the ribbon, the carriage simply moves in the direction of vector 18, i.e., across the ribbon along a line 7 perpendicular to the ribbon's direction of motion, at a speed Sscore given by:Sscore=Scarriage cos α.
As described in the '994 publication, a light-emitting device that provides a laser light beam and a nozzle that provides a stream of a cooling fluid (e.g., water) are coupled to the carriage and together form a vent across the width of the ribbon as the carriage moves along the linear track. Also coupled to the carriage is a mechanical scoring head (e.g., a scoring wheel) for forming an initiation flaw in the glass ribbon.
FIG. 4 schematically illustrates these aspects of the '994 publication, where reference numbers 21, 22, and 23 represent the locations at the beginning of the scoring process of (1) the footprint of the cooling fluid, (2) the footprint of the laser light beam, and (3) the initiation flaw, and reference numbers 31 and 32 represent the locations of the footprint of the cooling fluid and the footprint of the laser light beam at a later point in time, after initiation has been completed and the mechanical scoring head is no longer in contact with the ribbon.
As discussed in the '994 publication, to avoid producing a long initiation flaw, the time during which the mechanical scoring head is in contact with the glass ribbon needs to be short. This, in turn, means that the mechanical scoring head has to move towards and then away from the surface of the ribbon at high speed. Although such high speed motion can be achieved, the equipment required is generally complex and expensive. The present disclosure is directed to apparatus and methods for forming an initiation flaw with a mechanical scoring head using reduced speeds for the motions of the head towards and/or away from the surface being scored.